Diesel Explorer: Choosing a chassis

Having settled in on the Cummins B3.3 for an engine, the next step was to choose
a vehicle to put it in. I had some rough requirements in mind:

Seating capacity: At least 4 people

Air conditioning and decent acoustics/sound system

4-wheel drive

Since the original inspiration for the project was a Cummins B3.3 in a Jeep
Wrangler
(YJ), that
was an obvious candidate, but I also had an affinity for and experience with
Ford body styles and interiors. The decision came down to two potential chassis:
Wrangler vs. Explorer. Both satisfied the basic requirement above, but there
were some more criteria to consider—fitment, noise and comfort, emissions,
initial cost, and fuel economy.

Fitment

An obvious requirement was that the engine fit inside the vehicle’s engine bay.
This contest goes to the Wrangler. The engine bay is huge, and the hood opens
very wide. With the Explorer, a body lift would most likely be necessary.

Winner: Wrangler

Noise/acoustics and overall comfort

The fun and excitement of the Jeep’s removable top comes along with a noisy
ride, and poor acoustics. For me, the Explorer’s acoustics and comfort were
worth the somewhat less hip styling. Electric windows and locks were icing on
the cake.

Winner: Explorer

Initial cost

On the used market, Wranglers fetch about 2–3 times the amount Explorers do.
Spending less on a chassis would save more for the engine, giving the Explorer
the edge.

Winner: Explorer

Emissions

Since this vehicle would likely throw many error codes with an OBD-II computer,
it was important to find a vehicle that was OBD-I. The TJ body style is not
available in the OBD-I years, while the 2nd generation Explorer body style was
just introduced in 1995, barely making the cutoff. The Explorer body style and
interior both beat the YJ, as discussed above, so the Explorer gets the nod
here.

Winner: Explorer

Fuel economy

The most important metric—and the main goal of the project—was to achieve high
fuel efficiency. Thus, this metric would likely decide the contest. Properties
for the two vehicles are tabulated below. On one hand, the Wrangler is lighter
than the Explorer, requiring less energy to get up to speed. However, the
Wrangler is also less aerodynamic, consuming more energy while cruising. To
compare the two, a typical driving scheme was devised, for which rough estimates
for fuel consumption were calculated. This driving scheme included

Accelerating 0–40 mph in 10 seconds

Cruising at 40 mph for 5 miles

Explorer

Wrangler

Mass (kg)

1806 [1]

1331 [2]

Drag coefficient

0.43 [3]

0.55 [3]

Frontal area ($\text{m}^2 $)

2.3

2.3

Calculating fuel consumption during cruising

This was the easy part. Neglecting friction in the tires and drivetrain—which
would be more or less equivalent between both vehicles—energy consumption
during cruising can be estimated by integrating the drag force over the distance
traveled.

Drag force is estimated using the formula

where $ \rho $ is the density of air, $ A_f $ is the vehicle’s frontal
area (approximately equal for both Explorer and Wrangler), $ C_D $ is the
vehicle’s drag coefficient, and $ V $ is the cruising speed. Since speed is
constant, the integration simplifies to multiplication, giving (in Nm)

Calculating fuel consumption during acceleration

Calculating the energy required during acceleration included consideration of
both drag and inertial forces. Since drag is not constant during acceleration,
we will use a more general method and integrate the varying power over the
acceleration time, which is simply the force multiplied by velocity, or

Integrating the constant acceleration gives the vehicle speed as a function of
time, i.e.,

Boiling this all down and substituting all but the variables that differ between
the vehicles, we obtain

The table below summarizes the results for both vehicles. As you can see, these
estimates show the Explorer consuming 16% less energy than the Wrangler, making
it a better choice with regards to fuel economy.

Explorer

Wrangler

Energy consumed during cruising (Nm)

$ 1.53 \times 10^6 $

$ 1.95 \times 10^6 $

Energy consumed during acceleration (Nm)

$ 2.98 \times 10^5 $

$ 2.24 \times 10^5 $

Total energy consumed (Nm)

$ 1.83 \times 10^6 $

$ 2.17 \times 10^6 $

Winner: Explorer

Conclusions

Obviously, since this post is in a series about a diesel Explorer, you can guess
which chassis won out in the end. The Wrangler is a cool design for sure, but
the Explorer’s acoustics, comfort, and most importantly aerodynamics made it my
choice.